It is necessary in remedial work in a cased oil well to mill out a portion of the casing that was previously cemented in the well. For example, when repairing a section of the pre-existing casing must be milled out. Milling tools are believed well known. The present disclosure is directed to an improved milling machine which is initiated in operation by an increase in mud pressure when installed on a drill string in a casing. It is constructed with an internal telescoping mandrel responsive to the increase in pressure. The pressure moves the mandrel relatively upward in the tool against a compressible coil spring, thereby setting the tool. Setting is accomplished by deflecting outwardly a set of pivotal cutting blades. They are jointly retracted radially inwardly until mandrel movement whereupon they move jointly outwardly. Outward movement is constrained by the surrounding casing; as the tool is rotated, the cutting blades cut into the surrounding casing, and ultimately penetrate the casing. As this occurs, further movement upwardly holds the cutters in the outwardly extended position. they are locked outwardly by the further movement of the internal mandrel. The tool is then lowered continuously during milling until a sufficient length of the casing has been milled away.
It may be necessary to mill away more casing than the cutters can tolerate. As this occurs, it is ordinarily necessary to retrieve the string of drill pipe, remove the milling tool and replace it with a new milling tool or at least replace the worn cutters with new cutters. Thereafter, it would be necessary to reposition the equipment previously in the casing at the depth where the previous milling job had been partially completed so that the next milling cut could then be continued. If the pipe string has to be pulled and then placed back in the well, substantial rig time is involved at a significant cost. Moreover, there is always the problem of relocating the bottom shoulder of the cut portion of the casing so that the next milling cut proceeds previously below the prior milling cut. Another problem relates to the burs left after cutting; they may puncture packers inserted later. By contrast, the present apparatus is a system wherein first and second or additional sets of cutters are installed on a common mechanism. The first set of cutters is retracted adjacent a mandrel to be deflected outwardly as described above. The same mandrel positions the second set of cutting blades which are not extended fully radially outwardly while the first set is extended. The first set of blades is thus used to make the first milling cut; when those blades wear away and milling progress becomes unacceptably slow, the pipe string is lifted slightly to move the second set of cutters upwardly until they are positioned adjacent the shoulder of the cut portion at which occasion the second cutters are free to deflect radially outwardly into the cutting position. The second set is the operative and is able to complete the milling without requiring intermediate retrieval of the pipe string. The multiple sets of cutters can be designed differently so that the first set of cutters simply cuts through the casing while the last set of cutters forms a chamfered shoulder and removes burs.
While the foregoing has spoken generally of at least one important advantage of the present apparatus, the detailed description of the milling tool is set forth below in conjunction with the drawings of the two preferred embodiments.